Classifying bottom with PSV, E1, E2 the output is a lower limit and upper limit (presumably amplitude of the echo?) and then a class which has a number like 789 or 791 or something.

I have been trying to find some documentation detailing this data, but I can't find anything.

Is there a "class number to text" translation somewhere? Why do they start at 780-something instead of 1? Can I use lower-limit / upper-limit to get the results comparable between different transducers? Is the algorithm open so I can figure out what is done and that way get a way to decide why different transducers give different results?

You can read more about bottom composition here: www.reefmaster.com.au/reference2/index.htm

The two values you are seeing in the map layers are the upper and lower range of each hardness area, much like the upper and lower depth range of an isobath. To get a better view of the hardness values calculated for each sonar log, look in the Bottom Classification panel of the track edit window. Here you will see the upper and lower range for each layer type along with a histogram of returns. The scale and offset for an individual track can be adjusted to try and match tracks recorded with different setups, but this process is a little hit and miss and I would not recommend trying to combine hardness data from different sources.

I have been nagging Matt for a few years to replace those numbers with the "index" of the layer, those numbers doesn't say much.

With hardness it is best not to mix transducers as Matt wrote.